If you go by the CSO statistics, I would say 3% for 2005 (based on CPI), but I'm not sure if this is the same as the 'cost of living'. Ask how 'cost of living' will be defined.
I wouldn't accept a cost of living increase in the present climate. What's the cost of living to do with the rent? It's all about supply and demand and presently there's a lot of rental accomodation supply!
There's been a recent thread on rent increases which may be of interest to you
I'm assuming your lease is for private accomodation